1. Field of the Invention
The present invention relates to novel ester compounds having alicyclic and oxirane structures which form polymers useful as a base resin to formulate a chemical amplification type resist composition adapted for microfabrication, and a method for preparing the same.
2. Prior Art
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 xcexcm or less.
The resist materials for use in photolithography using light of an excimer laser, especially ArF excimer laser having a wavelength of 193 nm, are, of course, required to have a high transmittance to light of that wavelength. In addition, they are required to have an etching resistance sufficient to allow for film thickness reduction, a high sensitivity sufficient to eliminate any extra burden on the expensive optical material, and especially, a high resolution sufficient to form a precise micropattern. To meet these requirements, it is crucial to develop a base resin having a high transparency, rigidity and reactivity. None of the currently available polymers satisfy all of these requirements. Practically acceptable resist materials are not yet available.
Known high transparency resins include copolymers of acrylic or methacrylic acid derivatives and polymers containing in the backbone alicyclic compounds derived from norbornene derivatives. All these resins are unsatisfactory. For example, copolymers of acrylic or methacrylic acid derivatives are relatively easy to increase reactivity in that highly reactive monomers can be introduced and acid labile units can be increased as desired, but difficult to increase rigidity because of their backbone structure. On the other hand, the polymers containing alicyclic compounds in the backbone have rigidity within the acceptable range, but are less reactive with acid than poly(meth)acrylate because of their backbone structure, and difficult to increase reactivity because of the low flexibility of polymerization. Additionally, since the backbone is highly hydrophobic, these polymers are less adherent to substrates. Therefore, some resist compositions which are formulated using these polymers as the base resin fail to withstand etching although they have satisfactory sensitivity and resolution. Some other resist compositions are highly resistant to etching, but have low sensitivity and low resolution below the practically acceptable level.
An object of the invention is to provide a novel ester compound having alicyclic and oxirane structures useful as a monomer to form a polymer for use in the formulation of a photoresist composition which exhibits improved adhesion and transparency when processed by photolithography using light with a wavelength of less than 300 nm, especially ArF excimer laser light as the light source. Another object is to provide a method for preparing the ester compound.
We have found that ester compounds of the general formula (1) can be prepared in high yields by a simple process to be described later; that a polymer obtained using the ester compound has high transparency at the exposure wavelength of an excimer laser; and that a resist composition using the polymer as a base resin is fully adherent.
In one aspect, the invention provides an ester compound of the following general formula (1). 
Herein R1 is hydrogen or methyl, R2 is a tertiary alkyl group of 4 to 20 carbon atoms, and k is 0 or 1.
Preferably the ester compound of formula (1) is represented by the following general formula (2) or (3). 
Herein R1 is hydrogen or methyl, R3, R4, R5, and R6 are independently straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms, and Z is a divalent hydrocarbon group of 4 to 15 carbon atoms which forms a ring with the carbon atom to which it is connected at both ends.
From a process aspect, the ester compound of the general formula (1) is prepared by effecting Darzen""s reaction between a carbonyl compound of the following general formula (4) and a haloacetate compound of the following general formula (5) in the presence of a base according to the following reaction scheme. 
Herein R1, R2 and k are as defined above and X is a chlorine or bromine atom.
The ester compounds of the invention are of the following general formula (1). 
Herein R1 is hydrogen or methyl, R2 is a tertiary alkyl group of 4 to 20 carbon atoms, and k is 0 or 1.
As the partial structure OR2, those of the following general formulas are preferred. 
Herein R3, R4, R5, and R6 are independently straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms. Z is a divalent hydrocarbon group of 4 to 15 carbon atoms such as a straight, branched or cyclic alkylene or alkenylene group, which forms a ring with the carbon atom to which it is connected at opposite ends.
Examples of the C1-15 alkyl groups represented by R3, R4, R5, and R6 include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, n-hexyl, cyclopentyl, cyclohexyl, methylcyclopentyl, ethylcyclopentyl, methylcyclohexyl, ethylcyclohexyl, bicyclo[2.2.1]heptyl, bicyclo[2.2.2]octyl, bicyclo[3.3.1]-nonyl, bicyclo[4.4.0]decanyl, tricyclo[5.2.1.02,6]decanyl, tetracyclo[4.4.0.12,5.17,10]dodecanyl, and adamantyl. Examples of the rings that the divalent hydrocarbon group Z forms with the carbon atom include cyclopentane, cyclopentene, cyclohexane, cyclohexene, bicyclo[2.2.1]heptane, bicyclo[4.4.0]decane, tricyclo[5.2.1.02,6]decane, tetracyclo[4.4.0.12,5.17,10]dodecane, and adamantane.
Of the ester compounds of formula (1), preferred are those of the following general formula (2) or (3). 
Herein R1, R3, R4, R5, R6 and Z are as defined above.
Illustrative, non-limiting, examples of the ester compounds of the formula (1) and especially formula (2) or (3) include those of the following structures. 
The ester compound of the invention can be synthesized, for example, by effecting Darzen""s reaction, also known as Darzen""s condensation, between a carbonyl compound of the following general formula (4) and a haloacetate compound of the following general formula (5) in the presence of a base according to the following reaction scheme. 
Herein R1, R2 and k are as defined above and X is a chlorine or bromine atom.
The bases used herein include metal amides such as sodium amide, potassium amide, lithium diisopropylamide, potassium diisopropylamide, lithium dicyclohexylamide, potassium dicyclohexylamide, lithium 2,2,6,6-tetramethyl-piperidine, lithium bistrimethylsilylamide, sodium bistrimethylsilylamide, potassium bistrimethylsilylamide, lithium isopropylcyclohexylamide, and bromomagnesium diisopropylamide: alkoxides such as sodium methoxide, sodium ethoxide, lithium methoxide, lithium ethoxide, lithium tert-butoxide, and potassium tert-butoxide; inorganic hydroxides such as sodium hydroxide, lithium hydroxide, potassium hydroxide, barium hydroxide, and tetra-n-butylammonium hydroxide; inorganic carbonates such as sodium carbonate, sodium hydrogen carbonate, lithium carbonate and potassium carbonate; metal hydrides such as sodium hydride, lithium hydride, potassium hydride, and calcium hydride; and alkyl metal compounds such as trityl lithium, trityl sodium, trityl potassium, methyl lithium, phenyl lithium, sec-butyl lithium, tert-butyl lithium, and ethyl magnesium bromide, but are not limited thereto.
Upon reaction, the molar ratio of the compounds of formulas (4) and (5) may be selected as appropriate although it is preferred to use the compound of formula (5) in an amount of about 0.5 to 1.5 moles, especially about 0.8 to 1.2 moles per mole of the carbonyl compound of formula (4). It is also preferred to use the base in an amount of about 0.5 to 2.0 moles, especially about 0.8 to 1.5 moles per mole of the carbonyl compound of formula (4).
Useful solvents are ethers such as tetrahydrofuran, diethyl ether, di-n-butyl ether, and 1,4-dioxane, hydrocarbons such as n-hexane, n-heptane, benzene, toluene, xylene and cumene, alcohols such as methanol, ethanol, isopropyl alcohol and tert-butyl alcohol, amines such as liquid ammonia and methylamine, and aprotic polar solvents such as dimethyl sulfoxide and N,N-dimethylformamide. Depending on reaction conditions, a choice may be made among these solvents alone and mixtures thereof. There may also be subordinately used any of compounds having ligands such as N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine (TMEDA), hexamethylphosphorous triamide (HMPA), N,Nxe2x80x2-dimethylpropyleneurea (DMPU) and 1,3-dimethyl-2-imidazolidinone (DMI).
Where inorganic hydroxides or inorganic carbonates are used as the base, reaction may be effected in a concomitant system of the aforementioned organic solvent and water, that is, two-layer system. In this case, a phase transfer catalyst such as a quaternary ammonium salt or quaternary phosphonium salt may be added for promoting or accelerating the reaction.
The reaction temperature depends on other reaction conditions. When a strong base is employed in an organic solvent, the reaction favors cooling at a temperature between xe2x88x9278xc2x0 C. and 10xc2x0 C. In the case of two-layer system reaction, the temperature usually ranges from ice cooling to room temperature, and even heating up to about 60xc2x0 C. is acceptable.
From the reaction mixture, the desired compound is obtained by a conventional aqueous work-up step. If necessary, the desired compound is purified by any conventional technique such as distillation, chromatography or recrystallization.
A polymer is prepared using the inventive ester compound as a monomer. One common procedure is by mixing the monomer with a solvent, adding a catalyst or polymerization initiator, and effecting polymerization reaction while heating or cooling the system if necessary. This polymerization reaction can be effected in a conventional way.
A resist composition is formulated using as a base resin the polymer resulting from polymerization of the ester compound. Usually, the resist composition is formulated by adding an organic solvent and a photoacid generator to the polymer and if necessary, further adding a crosslinker, a basic compound, a dissolution inhibitor and other additives. Preparation of the resist composition can be effected in a conventional way.
The resist composition formulated using the polymer resulting from polymerization of the inventive ester compound lends itself to micropatterning with electron beams or deep-UV rays since it is sensitive to high-energy radiation and has excellent sensitivity, resolution, and etching resistance. Especially because of the minimized absorption at the exposure wavelength of an ArF or KrF excimer laser and firm adhesion to the substrate, a finely defined pattern having sidewalls perpendicular to the substrate can easily be formed. The resist composition is thus suitable as micropatterning material for VLSI fabrication.